The invention arose during development efforts in marine racing applications. A high performance or racing cam shaft requires a higher than normal idle speed, e.g., 1,200 rpm. However, marine transmissions typically require a lower idle speed, e.g., 600 rpm, in order to shift into gear. When the racing engine is attempted to be idled at 600 rpm, it breaks into severe oscillation, with idle speed varying between 400 and 1,200 rpm, and finally not idling at all.
The present invention addresses and solves the above noted problem by controlling spark timing, including a radical reduction in spark timing from a normal 10.degree. before top dead center to 15.degree.-20.degree. after top dead center and by providing a negative slope timing curve for self-stabilization. As engine speed increases, so does the amount of retard, which in turn reduces engine speed and thus stabilizes same. Likewise, as engine speed decreases, the amount of retard decreases and advances engine timing to thus increase engine speed, again providing stabilization. A smooth idle around 600 rpm is achieved. At engine speeds above a predetermined value, e.g., 900 rpm, the timing curve transitions to a positive slope and at a steeper rate to quickly advance engine timing and merge with a baseline timing curve.
The timing delay of ignition pulses relative to a top dead center position of the engine increases with increasing engine speed along a negative slope to a maximum delay at a predetermined engine speed, e.g., 900 rpm, and the relative timing delay then decreases along a steeper positive slope at higher engine speeds to merge with the baseline timing curve.
An engine temperature sensitive circuit is provided to reduce the retard and advance engine timing to the baseline curve when the engine is cold, and to retard the relative engine timing from the baseline curve at idle after the engine reaches a given temperature. A starting circuit is included to reduce the retard and advance the engine timing during cranking regardless of engine temperature. An acceleration detection circuit senses throttle position and quickly advances engine timing to eliminate the retard and enable quick acceleration when called for by throttle advancement. A disabling circuit disables the acceleration detection circuit at high engine speeds so that engine timing follows only the baseline curve.